Over-Current Protection Circuit And Method

ABSTRACT

A determination is made as to when the current flowing through a transistor exceeds a predetermined threshold. When the current exceeds the predetermined threshold, the transistor is deactivated. The deactivating of the transistor is effective to limit the current that flows through the transistor. The limiting of the current is effective to prevent damage to the transistor in an over current condition. The transistor is maintained in a deactivated state until a time off circuit resets the DC-DC converter circuit. The maintaining of the transistor in the deactivated state until a time off circuit resets the DC-DC converter circuit is additionally effective to reduce the time on (Duty Cycle—D.C.) and frequency to further prevent damage to the transistor due to switching power losses.

TECHNICAL FIELD

This application relates to DC-DC converters and, more specifically, for protection circuits for transistors in DC-DC converters.

BACKGROUND OF THE INVENTION

Batteries (and sometimes multiple batteries) are used to power various types of systems. Some examples of battery usage are when batteries are used to power the electrical system of a conventional internal combustion vehicles, the motor of electric vehicles or hybrid electric vehicles. The batteries in these systems can be packs of batteries that provide voltages in the 12-400 volts range.

Accessories and other components (e.g., fuel injectors) that are used in the same vehicles typically require voltages that are different than 200-400 volts. These voltages are typically provided from a 12V battery or a battery array of 24V. .One such conversion system that can be used is a DC-DC converter. In some examples of DC-DC converters, the voltage may be converted from 12 volts to 48 volts (for example, to drive fuel injectors). Other examples of conversions are possible.

Although DC-DC converters have been used, they have operational issues and limitations. For instance, the conversion circuitry in the DC-DC converter typically utilizes transistors. Transistors can be damaged or destroyed when too high a current is applied. For example, in certain applications that use inductors, the inductors can become defective introducing high currents into the transistors that drive the circuits. In other examples, overloads at the output or glitches in the control logic of the circuit can result in too much current flowing through transistors, possibly leading to the destruction of these transistors.

Previous approaches have not been entirely successful in solving the above-mentioned problems. This has resulted in some user dissatisfaction with these previous systems and approaches.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the disclosure, reference should be made to the following detailed description and accompanying drawings wherein:

FIG. 1 comprises a circuit diagram of a protection circuit according to various embodiments of the present invention;

FIG. 2 comprises a circuit diagram of a system that utilizes the protection circuit of FIG. 1 according to various embodiments of the present invention;

FIG. 3 comprises a graph show aspects of the operation of FIG. 1 and FIG. 2 according to various embodiments of the present invention.

Skilled artisans will appreciate that elements in the figures are illustrated for simplicity and clarity. It will further be appreciated that certain actions and/or steps may be described or depicted in a particular order of occurrence while those skilled in the art will understand that such specificity with respect to sequence is not actually required. It will also be understood that the terms and expressions used herein have the ordinary meaning as is accorded to such terms and expressions with respect to their corresponding respective areas of inquiry and study except where specific meanings have otherwise been set forth herein.

DETAILED DESCRIPTION

Approaches are described herein that halt the operation of any one or more channels of a high voltage DC-DC converter at a time when the current of more than a predetermined threshold is reached. In particular, a driver transistor that is used by any of the channels is deactivated so that the transistor may not be harmed by an over-current condition. The transistor remains off for the entirety of a fixed time out assuring that the transistor survives the over-current condition.

In many of these embodiments, a determination is made as to when the current flowing through the main transistor exceeds a predetermined threshold. When the current exceeds the predetermined threshold, the transistor is deactivated. The deactivation of the transistor is effective to limit the current that flows through the main switch and significantly reduce its power dissipation . . . The limiting of the current is effective to prevent damage to the transistor in an over current condition. The transistor is maintained in a deactivated state until a time out circuit resets the DC-DC converter. The maintaining of the transistor in the deactivated state is additionally effective to significantly reduce the duty cycle (time on) and frequency of operation of the main switch and thus reduce its power dissipation.

In yet other aspects, the over current condition could be caused by a defective inductor in the DC-DC converter circuit. One way to detect this over current condition is by comparing a reference voltage against a voltage proportional to the current flowing through the transistor. This comparison is made using an electronic circuitry arranged as a comparator. The output of the comparator is used to reset the channel memory latch and deactivate the main switch. The output of the latch could also be used as a fault condition indication to a microcontroller.

In some aspects, the DC-DC converter drives a device on the vehicle. In some examples, the device is a fuel injection apparatus. Other examples of devices are possible.

Referring now to FIG. 1 and FIG. 2, one example of a protection circuit 102 disposed within a DC-DC converter 100 is described. The protection circuit 102 includes a first resistor 104 (R1999), a second resistor 105 (R2000) an operational amplifier 106 (U3) a first capacitor (C1) 107, a third resistor 111 (2001) a second operational amplifier (U2) 108, a first NAND gate 109, and a second NAND gate 110. The NAND gates 109 and 110 are arranged as a latch.

The DC-DC converter 100 has an input circuit 120, an output circuit 130, a driver circuit 140, a time off circuit 150, a current feedback circuit 160, a channel memory 170, a transistor 190, a NAND gate 192, a bi-polar transistor 93, a resistor 194, and a capacitor 195.

The input circuit 120 includes a battery 122, a capacitor 124, a resistor 126, and an inductor 128. The output circuit 130 includes a capacitor 132, a switch 134, and a load 136. The load 136 may be a vehicle accessory in one example. In another example, the load may be a fuel injector. Other examples are possible.

The driver circuit 140 includes a first transistor 141, a second transistor 142, and a third transistor 143, a capacitor 144, and resistors 145, 146, and 147. The time off circuit 150 includes an operational amplifier 151, a resistor 152, and capacitors 153 and 154.

The current feedback circuit 160 includes resistors 161, 162, and 163, and capacitors 164 and 165, and an operational amplifier 166. The channel memory 170 includes NAND gates 171, 172, 173, and 174.

The transistor 190 is connected as an inverter and coupled to the NAND gate 192, the protection circuit 102, the input circuit 120, and the output circuit 180.

In some aspects of the operation of the circuits of FIG. 1 and FIG. 2, the battery 122 produces a voltage and current. The current flows through and is stored in the inductor 128. Current from the inductor 128 goes to the driver circuit 140 when the transistor 143 is closed, this current flows to the shunt resistor 161 creating a voltage at the node 167 in the current feedback circuit 160. When this voltage exceeds a predetermined value, the output of the operational amplifier 166 in the current feedback circuit 160 change to a logical low value.

The low value at the output of the operational amplifier 166 resets the latch in the channel memory 170. In particular, the output of NAND gate 172 changes to high logical level, the output of the next NAND gate 173 changes to low logical level, and the output of the next NAND gate 174 changes to high logical level if the other input of the gate 174 is set to high logical level (the enable input).

This high logical level from the NAND gate 174 is transmitted back to the driver circuit 140. This, in turn, activates the transistor 142. Activation of the transistor 142 turns off the driver transistor 143. Consequently, current no longer goes through the path from the inductor 128 to the current feedback circuit 160. Instead, this current goes out to the output 130.

If the inductor 128 has zero current, negative voltage at the output of the inductor (this condition is called “swing back”), the diode 193 is forward biased, and this allows the capacitor 153 to discharge.

The protection circuit 102 in this example is configured to have several stages and in the first stage the amplifier 106 may be an LM2903 comparator that is configured as a non-inverting amplifier in one example. Vshunt is connected to the current sensing shunt of the DC/DC converter that is the courses through the driver transistor 143. This stage amplifies the voltage of the shunt to about 11 times. Other amplifications are possible.

The second stage of the protection circuit in this example is the comparator 108, when the amplified Vshunt crosses the threshold of 2.5V reference voltage the output turns to 0V and is maintained at 5V otherwise. Other values are possible.

The third stage of the protection circuit 102 is an S-R latch made by two fast switching NAND gates (NAND gates 109 and 110). When the output of comparator 108 turns to 0V the latch is reset causing the outputs (Q and Q) to switch logical states. Q is also referred to as “Q negative” herein.

Q is normally in high state while Q is in low state. When an over current condition is detected, Q turns to low state and Q turns to high. Q is connected to the transistor 190 (which in one example is a NMOS that activates/deactivates the “swing back” detection). When Q is in high state, the transistor 190 is closed and the “swing-back” detection is activated thus the DC/DC converter 100 operates normally. When Q is low the transistor 190 opens thereby deactivating the “swing back” detection. This effectively cuts the frequency of operation of the circuit of FIG. 2 to a minimum. Thus, this forces the driver circuit 140 to remain off for as long as the voltage at the RC network (capacitor 153 and resistor 152) of the time off circuit 150 is above the threshold (e.g., 2.5 volts).

Q is connected to a transistor 197 (e.g., a digital NPN), which can reset the latch of channel memory 170, which in turn will turn off the driver transistor 143. This effectively limits the maximum current that can run through the transistor 143 and the inductor 128. Finally and when the time-out comparator of the DC/DC converter circuit changes to 0V, the latch in the channel memory 170 is set again and everything is ready for the next cycle.

Eventually, the time out circuit 150 changes to have a zero output. This sets the latch in the memory circuit 170 and a new cycle of operation is initiated.

Finally and when the output of the time-off comparator 151 changes to 0V, the latch (NAND gate 110) in the protection circuit 102 is set again and all components of the DC-DC converter 100 are ready for the next cycle.

Referring now to FIG. 3, examples of applying the present approaches are described. The example of FIG. 3 utilizes two circuits (i.e., channels) with each channel having a circuit of FIG. 2. A first current 302 (from the inductor 128) of the first channel is the proper behavior of a non defective inductor. However, the second inductor is defective and produces the current waveform 304. This is limited by the comparison of the waveform 306 against the threshold 308 because of the operation of the protection circuit 102. If no protection circuit 102 existed, the current waveform 304 would present an over current and the transistor 143 would become damaged or destroyed. A waveform 306 shows the output of non negative amplifier 106. The waveform 310 is part of the waveform 304 and represents the time when the transistor remains off, even though; the current in the defective coil falls to zero Amperes.

Preferred embodiments of this invention are described herein, including the best mode known to the inventors for carrying out the invention. It should be understood that the illustrated embodiments are exemplary only, and should not be taken as limiting the scope of the invention. 

What is claimed is:
 1. A method of preventing current overload in a transistor that is used in a DC-DC converter circuit, the method comprising: determining when the current flowing through the transistor exceeds a predetermined threshold and when the current exceeds the predetermined threshold, deactivating the transistor, the deactivation of the transistor is effective to significantly limited the current that flows through the transistor, the limiting of the current being effective to prevent damage to the transistor in an over current condition; Maintaining the transistor in a deactivated state until a time off circuit resets the DC-DC converter circuit, being additionally effective to reduce the time on (Duty Cycle—D.C.) and frequency to further prevent damage to the transistor due to switching power losses.
 2. The method of claim 1, wherein the DC-DC converter drives a device on the vehicle.
 3. The method of claim 2, wherein the device is a fuel injection apparatus.
 4. The method of claim 1, wherein the determining whether the current going through the transistor exceeds a predetermined threshold comprises comparing a voltage associated with the transistor to a voltage threshold using a comparator.
 5. The method of claim 1, further comprising sending a fault condition indication to a controller.
 6. The method of claim 1, wherein the over current condition is caused by a defective inductor in the DC-DC converter circuit.
 7. The method of claim 1, wherein deactivating the transistor comprises reset a channel memory latch.
 8. An apparatus that is configured to prevent current overload at a transistor that is used in a DC-DC converter circuit, the apparatus comprising: a protection circuit that determines when the current flowing through the transistor exceeds a predetermined threshold and when the current exceeds the predetermined threshold, functions to deactivate the transistor, the deactivating of the transistor being effective to significantly reduce the frequency of operation and/or D.C. of the DC-DC converter circuit, the deactivating of the transistor limiting the current that flows through the transistor, the limiting of the current being effective to prevent damage to the transistor in an over-current condition; such that the transistor is maintained in a deactivated state by the protection circuit until a time off circuit resets the DC-DC converter circuit, the maintaining of the transistor in the deactivated state being additionally effective to prevent damage to the transistor from an over current condition.
 9. The apparatus of claim 8, wherein the DC-DC converter drives a device on the vehicle.
 10. The apparatus of claim 9, wherein the device is a fuel injection apparatus.
 11. The apparatus of claim 8, wherein the determination of whether the current going through the transistor exceeds a predetermined threshold is performed by comparing a reference voltage against a voltage proportional to the current flowing through the transistor.
 12. The apparatus of claim 8, wherein a fault condition indication to a controller.
 13. The apparatus of claim 8, wherein the over current condition is caused by a defective inductor in the DC-DC converter circuit.
 14. The apparatus of claim 8, wherein deactivation of the transistor is effective to reset a channel memory latch. 